The recovery of hydrocarbons from subterranean zones relies on the process of drilling wellbores. This process includes drilling equipment situated at surface and a drill string extending from the surface equipment to the formation or subterranean zone of interest. The drill string can extend thousands of feet or meters below the surface. The terminal end of the drill string includes a drill bit for drilling, or extending, the wellbore. The process also relies on some sort of drilling fluid system, in most cases a drilling “mud”. The mud is pumped through the inside of the drill string, which cools and lubricates the drill bit and then exits out of the drill bit and carries rock cuttings back to surface. The mud also helps control bottom hole pressure and prevents hydrocarbon influx from the formation into the wellbore and potential blow out at the surface.
Directional drilling is the process of steering a well from vertical to intersect a target endpoint or to follow a prescribed path. At the terminal end of the drill string is a bottom hole assembly (BHA) which may include 1) the drill bit; 2) steerable downhole mud motor of a rotary steerable system; 3) sensors of survey equipment for logging while drilling (LWD) and/or measurement while drilling (MWD) to evaluate downhole conditions as drilling progresses; 4) apparatus for telemetry of data to surface; and 5) other control equipment such as stabilizers or heavy weight drill collars. The BHA is conveyed into the wellbore by a string of metallic tubulars known as the drill string. MWD equipment may be used to provide downhole sensor and status information at the surface while drilling in a near real-time mode. This information is used by the rig crew to make decisions about controlling and steering the well to optimize the drilling speed and trajectory based on numerous factors, including lease boundaries, existing wells, formation properties, hydrocarbon size and location. These decisions can include making intentional deviations from the planned wellbore path as necessary, based on the information gathered from the downhole sensors during the drilling process. In its ability to obtain real time data, MWD allows for a relatively more economical and efficient drilling operation.
In known MWD systems, the MWD tools typically contain the same sensor package to survey the well bore, but various telemetry methods may be used to send the data back to the surface. Such telemetry methods include, but are not limited to, the use of hardwired drill pipe, acoustic telemetry, use of fibre optic cable, mud pulse (MP) telemetry and electromagnetic (EM) telemetry.
MP Telemetry involves creating pressure pulses in the circulating drill mud in the drill string. Mud is circulated from the surface to downhole using positive displacement pumps. The resulting flow rate of mud is typically constant. Pressure pulses are generated by changing the flow area and/or flow path of the drilling mud as it passes the MWD tool in a timed, coded sequence, thereby creating pressure differentials in the drilling mud. The pressure pulses act to transmit data utilizing a number of encoding schemes. These schemes may include amplitude phase shift keying (ASK), frequency shift keying (FSK), phase shift keying (PSK), or a combination of these techniques.
The pressure differentials or pulses may either be negative pulses or positive pulses. Valves that open and close a bypass mud stream from inside the drill pipe to the wellbore annulus create a negative pressure pulse. All negative pulsing valves need a high differential pressure below the valve to create a sufficient pressure drop when the valve is open; this results in the negative valves being more prone to washing. With each actuation, the valve hits against the valve seat to ensure it completely closes the bypasses and this impact can lead to mechanical and abrasive wear and failure. Valves that use a controlled restriction within the circulating mud stream create a positive pressure pulse. Some valves are hydraulically powered to reduce the required actuation power typically resulting in a main valve indirectly operated by a pilot valve. The pilot valve closes a flow restriction which actuates the main valve to create a pressure change.
A number of different valves are currently used to create positive pressure pulses. In a typical rotary or rotating disc valve pulser, a control circuit activates a motor (e.g. a brushless, DC electric motor) that rotates a “windowed restrictor” or rotor, relative to a fixed housing (stator) to allow (open the window) or restrict (close the window) fluid flow through the restrictor. It is the variable alignment of the rotor and stator that produces the ‘windows of fluid flow’, and the movement between aligned (open) and misaligned (closed) that produces the pressure pulses. The rotor is rotated either continuously in one direction (mud siren), incrementally by oscillating the rotor in one direction and then back to its original position, or incrementally in one direction only, so that the rotor blades increase or decrease the amount by which they obstruct the windows in the stator. As the rotor rotates, it partially blocks a portion of the window, fluid becomes restricted causing a change in pressure over time. Generally, mud pulse valves are capable of generating discrete pulses at a predetermined frequency by selective restriction of the mud flow.
Rotary pulsers are typically actuated by means of a torsional force applicator which rotates the rotor a short angular distance to either open or close the pulser, with the rotor returning to its start position in each case. Motor speed changes are required to change the pressure pulse frequency. Various parameters can affect the mud pulse signal strength and rate of attenuation such as original signal strength, carrier frequency, depth between surface transducer and downhole modulator, internal diameter of the drill pipe, density and viscosity of the drilling fluid, volumetric flow rate of drilling mud, and flow area of window. Rotary valve pulsers require an axial gap between the stator and rotor of the modulator to provide a flow area for drilling mud, even when the valve is in the “closed” position. As a result the rotary pulser is never completely closed as the drilling mud must maintain a continuous flow for satisfactory drilling operations to be conducted. The size of the gap is dictated by previously mentioned parameters, and a skilled technician is required to set the correct gap size and to calibrate the pulser.
Another type of valve is a “poppet” or reciprocating pulser where the valve opens and closes against an orifice positioned axially against the flow stream. Some have permanent magnets to keep the valve in an open position. The permanent magnet is opposed by a magnetizing coil powered by the MWD tool to release the poppet to close the valve.
U.S. Pat. No. 8,251,160, issued Aug. 28, 2012, discloses an example of a MP apparatus and method of using same. It highlights a number of examples of various types of MP generators, or “pulsers”, which are familiar to those skilled in the art. U.S. Pat. No. 8,251,160 describes a rotor/stator design with windows in the rotor which align with windows in the stator. The stator also has a plurality of circular openings for flow of fluid therethrough. In a first orientation, the windows in the stator and the rotor align to create a fluid flow path orthogonal to the windows through the rotor and stator in addition to a fluid flow path through the circular openings in the stator. In this fashion the circulating fluid flows past and through the stator on its way to the drill bit without any significant obstruction to its flow. In the second orientation, the windows in the stator and the rotor do not align and there is restriction of fluid flow as the fluid can only flow through the circular holes in the stator. This restriction creates a positive pressure pulse which is transmitted to the surface and decoded.
Advantages of MP telemetry include increased depth capability, no dependence on earth formation, and current strong market acceptance. Disadvantages include many moving parts, difficulty with lost circulation material (LCM) usage, generally slower baud rates, narrower bandwidth, and incompatibility with air/underbalanced drilling which is a growing market in North America. The latter is an issue as the signals are substantially degraded if the drilling fluid inside the drill pipe contains substantial quantities of gas. MP telemetry also suffers when there are very low flow rates of mud, as low mud flow rates may result in too low a pressure differential to produce a strong enough signal at the surface. There are also a number of disadvantages of current MP generators, that include limited speed of response and recovery, jamming due to accumulation of debris which reduces the range of motion of the valve, failure of the bellows seal around the servo-valve activating shaft, failure of the rotary shaft seal, failure of drive shaft components, flow erosion, fatigue, and difficulty accesses and replacing small parts.